1. Field of the Invention
The present invention relates to a control circuit of a direct current-direct current (DC-DC) converter applied to a power supply such as a switching mode power supply (SMPS) or the like, and more particularly, to a control circuit of a DC-DC converter that can selectively control a logical link control (LLC) resonant half bridge converter as well as an active clamp forward (ACF) converter using a single control circuit capable of selectively supporting an ACF or LLC operating mode.
2. Description of the Related Art
In general, an SMPS is a device for achieving a DC output voltage from a DC input voltage, which is controlled through a filter after being converted into a square waveform voltage by a semiconductor device such as a metal-oxide-semiconductor field-effect transistor (MOSFET) for power as a switch. As compared to a conventional linear power supply, the SMPS is a stabilized power supply having advantages of higher efficiency, higher durability, smaller size, and lighter weight by controlling a power flow using a switching processor of a semiconductor device.
According to switching control modes in this SMPS, there are an ACF converter based on a half bridge and an LLC resonant half bridge converter recently receiving attention. To control each of these converters, a dedicated control device has been developed, which allows a plurality of dedicated integrated circuits (ICs) to operate MOSFETs for power within the SMPS.
For example, the ACF DC-DC converter may be considered to be a half bridge series converter since two active switches are used, and may have a power conversion capacity of up to 500 W as well known. The ACF DC-DC converter may perform soft switching to reduce the loss of switching, and may limit a voltage stress of a switch to improve reliability.
The LLC resonant half bridge converter basically has a half bridge series circuit, which can be designed to adapt to medium or large capacity. The LLC resonant half bridge converter is a power supply system, which is actually applicable to a capacity of 500 W or more and has been recently actively researched. Since the LLC resonant half bridge converter uses a resonance scheme, the loss of switching and the loss of conductivity may be reduced in principle. Accordingly, the LLC resonant half bridge converter is one of circuit systems having great latent potential capable of increasing the power conversion efficiency.
The SMPS plays a role in supplying a load with a constant DC output voltage. When an output voltage is reduced by load fluctuation due to parasitic elements of a component in the converter, it is difficult for the SMPS to be applied as a stabilized power supply. For this reason, a feedback control circuit capable of stabilizing the output voltage is required.
Representative examples of the feedback control circuit are a pulse width modulation (PWM) scheme and a pulse frequency modulation (PFM) scheme. The ACF DC-DC converter conventionally uses a PWM control circuit adopting the PWM scheme. In the LLC resonant half bridge converter, a PFM control circuit adopting the PFM scheme is used and an individual dedicated control device is required for control of each DC-DC converter.
Next, a control scheme of an existing DC-DC converter will be described in more detail.
Conventionally, the SMPS is a device for converting energy using an energy storage device such as an inductor, a transformer, a capacitor, or the like. There are a boost-up or step-up scheme for changing a low input voltage to a high voltage, a step-down scheme as opposed thereto, an inverting scheme for simply inverting the polarity, and the like.
Herein, the DC-DC converter is a circuit for receiving a DC input to obtain a constant DC output voltage by providing feedback suitable for a switching regulator. This circuit is disadvantageous in that noise or ripple occurs in the DC output voltage since energy is transferred in a discontinuous packet scheme every time.
To minimize this noise or ripple, components of an inductor, a capacitor, and the like should be properly selected and a proper control scheme should be used. Conventionally, basic control schemes are the PFM scheme and the PWM scheme as described above.
Using the PWM control scheme, the conventional ACF DC-DC converter generates a constant output voltage by generating a pulse signal whose duty varies with a voltage fed back from an output voltage, a reference voltage, and a pulse signal waveform of an oscillator and controlling a transistor serving as a switch.
In this case, a PWM control circuit controls a pulse width to be large when load is large, thereby constantly maintaining an output voltage.
A clamp circuit included in the DC-DC converter performs a transformer reset function, clamps a switch voltage, and enables a zero voltage switching (ZVS) operation of primary and auxiliary switches.
Using the PFM control scheme, the conventional LLC resonant half bridge converter generates a constant output voltage by turning on/off a transistor serving as a switch using an oscillator's pulse signal whose duty is constant at a fixed frequency, a voltage fed back from an output voltage, and a reference voltage.
In this case, a PFM control circuit may constantly maintain an output voltage by performing a control operation for constantly maintaining a pulse width and lowering a pulse frequency when load is small.
The DC-DC converter has an advantage of superior system efficiency and ensures zero voltage switching over the overall load region due to a large magnetizing current. In the LLC resonant half bridge converter, a gate signal applied to a MOSFET switch is complementary, a duty cycle is 50%, and a variable operating frequency is controlled to adjust an output voltage.
However, it is known that the conventional ACF DC-DC converter unstably operates in a light load situation. Thus, it is recommended to use the conventional ACF DC-DC converter in a heavy load situation.
It is known that the conventional LLC resonant half bridge converter unstably operates in the heavy load situation. Thus, it is recommended to use the conventional LLC resonant half bridge converter in the light load situation.
A control circuit for detecting a load state, controlling the LLC resonant half bridge converter in the PFM scheme for the light load situation, and controlling the ACF DC-DC converter in the PWM scheme for the heavy load situation is disclosed in Korean Patent Publication No. 2005-0007171.
The conventional DC-DC converters respectively require a dedicated control device. However, there is a problem in that a single dedicated device may not be applied to both the ACF DC-DC converter and the LLC resonant half bridge converter.
A dedicated control circuit for controlling each power mode and a control circuit operating in the PWM scheme for the heavy load and operating in the PFM scheme for the light load have been proposed. However, there is a problem in that the ACF DC-DC converter and the LLC resonant half bridge converter may not be supported in a single control device.